JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Thank you for contacting us. I have create a sample form that you may clone if you like
In this sample you you will notice that I added 3 number fields and a Form Calculation widget field
The Form Calculation enables to due simple calculation between fields
When you open the wizard, you may add the fields by pressing ADD FIELD and select the type of algorithm you wish. Once you have finished press the FINISH button to save changes.
Please let us know if this helps.